Currently, a satellite may receive a signal representing audio, video or data information from a transmitter. The satellite amplifies and broadcasts this signal to a receiver via a communication channel at a specified frequency and bandwidth. Since communications channels are subject to errors due to noise within the channel itself and to external sources, error correction is desirable. One technique for reducing or eliminating errors is Forward Error Correction (FEC). This technique sends a certain amount of extra information along with the original information. When errors occur, the receiver uses the extra information to locate and correct errors without further communication with the transmitter.
Two widely used types of forward error correction systems employ convolutional coding and block coding. Convolution coding operates on a continuum of data which is serially and continuously conveyed to an encoder. A convolutional encoder analyzes the current data and some amount of previous data. The encoder adds error correction data to the current data, thereby creating a new data signal. The system then outputs a continuous stream of the new data at a higher rate, either more data being transmitted faster, or more data over a longer time period. The receiver is conditioned to analyze a signal encoded with the convolutional error coding method used by the transmitter.
Block coding, such as Reed-Solomon coding, encodes the data signal with additional error correcting data using a specified algorithm. In a Reed-Solomon encoder, data is typically divided into equal sized units or blocks of a convenient size. Using a Reed-Solomon algorithm, these blocks have data added to them in some manner that is dependent upon the data itself. This process creates a new block of somewhat larger size that may or may not resemble the original data. However, by the receiver understanding the coding scheme used, the new block of data may be analyzed and the original data extracted, even if errors have been created in the data.
Each type of error coding has an associated code rate based on the number of bits entering the encoder divided by the number leaving it. Thus if 750 bits of data enter, 250 bits of error correction code are added, and 1000 bits (750+250) are output, the code rate is said to be a 3/4 (750/1000) code rate, and the apparatus is said to run at a 3/4 error correction coding rate. This is sometimes referred to as rate 3/4 forward error correction.
These same rates designate the error decoding rate used by a decoder, although the reverse operation is performed. For example, 1000 bits of data may be input, 250 of which are error correction code and the remaining 750 bits are data. The 250 bits of error correction code are removed from the data signal and used to detect and correct errors in the data signal. The remaining 750 bits of data are output. The decoder is said to run at a 3/4 error correction decoding rate.
The amount of error correcting information encoded into a data signal may depend, in part, on the operation of the satellite. For example, a satellite broadcast system may operate in two power modes, low and high. At high power, the signal received and transmitted by the satellite is stronger. As a result, the quality of a received signal is improved, and less error correction coding is required to obtain a desired data quality. For example, at high power, transmitted data may be approximately 25% error correction data and 75% usable data. Similarly, when the satellite operates at low power, the signal transmitted and received is weaker. Additional error correction data is therefore required to achieve the desired data quality. For example, at low power, approximately 40% of the transmitted data is error correction data and approximately 60% is usable data.
A desirable error correction coding rate will maximize the usable data and minimize the error correction data transmitted. If insufficient error correction data is included in the transmitted signal, the signal will not be received reliably, if at all, by the receiver. If excessive error correction data is included, the signal will be received correctly, but a lesser percentage of the output signal will be available for real data than if the error correction data had been matched to the transmitting power of the satellite.